There is a conventionally known refrigeration apparatus of the type which performs a refrigeration cycle by circulating refrigerant through a refrigerant circuit which is a closed circuit. Such a type of refrigeration apparatus has been used widely as an air conditioner or other like apparatus. For example, Japanese Patent Application Kokai Publication No. 2001-107881 discloses one such refrigeration apparatus in which the high pressure of a refrigeration cycle is set higher than the critical pressure of a refrigerant. This refrigeration apparatus includes, as a mechanism for expanding refrigerant, an expander formed by fluid machinery of the scrolled type. And, the expander is connected to a compressor by a shaft, with a view to accomplishing improvement in COP (coefficient of performance) by making utilization of power produced in the expander for driving the compressor.
In the refrigeration apparatus disclosed in the aforesaid gazette, the mass flow rate of refrigerant that passes through the expander becomes constantly equal to the mass flow rate of refrigerant that passes through the compressor. This is because the refrigerant circuit is formed by a closed circuit. On the other hand, both the density of refrigerant at the entrance of the expander and the density of refrigerant at the entrance of the compressor vary, depending on the operation condition of the refrigeration apparatus. In the refrigeration apparatus of the aforesaid gazette, however, the expander and the compressor are connected together, and it is impossible to make the ratio between the displacement volume of the expander and the displacement volume of the compressor variable. This gives rise to a problem that, when there are changes in operating condition, it becomes impossible for the refrigeration apparatus to continue to operate stably.
To cope with this problem, Japanese Patent Application Kokai Publication No. 2001-116371 proposes a technique of providing in the refrigerant circuit a bypass line that bypasses an expander. Stated another way, if the displacement volume of the expander is insufficient, a portion of refrigerant that has dissipated heat is made to flow into the bypass line for assuring the circulation amount of refrigerant, with a view to enabling a refrigeration cycle to continue in stable manner.
But in reality the displacement volume of the expander may become excessive depending on the operation condition of the refrigeration apparatus. Also in this case, it becomes impossible for the refrigeration apparatus to continue to operate stably. A measure for this problem is disclosed by Fukuda, Mitsuhiro and two others in a paper entitled “THEORETICAL PERFORMANCE OF CARBON DIOXIDE CYCLE WITH INCORPORATION OF COMPRESSOR/EXPANDER INTEGRATED TYPE FLUID MACHINERY”, 35th Air Conditioning and Refrigeration Combined Lecture Meeting, Lecture Collected Papers, pp. 57-60. More specifically, in this non-patent document, in order to deal with the problem, an expansion valve is disposed upstream of an expander in addition to a bypass line that bypasses the expander. To sum up, refrigerant traveling in the direction of the expander is decompressed by the expansion valve. That is, the specific volume of refrigerant flowing into the expander is increased beforehand, with a view to enabling a refrigeration cycle to continue in stable manner.